It has been recognized that crops grown in some soils are naturally resistant to certain fungal pathogens. Furthermore, soils that are conducive to the development of these diseases can be rendered suppressive, or resistant, by the addition of small quantities of soil from a suppressive field. Scher et al. Phytopathology 70:421 (1980). Conversely, suppressive soils can be made conducive to fungal diseases by autoclaving, indicating that the factors responsible for disease control are biological. Subsequent research has demonstrated that root colonizing bacteria are responsible for this phenomenon known as biological disease control (BDC). Baker et al., Biological control of plant pathogens, (Freeman Press, San Francisco) (1974).
In many cases, the most efficient strains of biological disease controlling bacteria are fluorescent pseudomonads. Weller et al., Phytopathology, 73:463-469 (1983). These bacteria have also been shown to promote plant growth in the absence of a specific fungal pathogen by the suppression of detrimental rhizosphere microflora present in most soils. Kloepper et al., Phytopathology 71:1020-1024 (1981). Important plant pathogens that have been effectively controlled by seed inoculation with these bacteria include Gaeumannomyces gramminis, the causative agent of take-all in wheat, Cook et al., Soil Biol. Biochem 8:269-273 (1976) and Pythium ultimum and Rhizoctonia solani, pathogens involved in damping off of cotton. Howell et al., Phytopathology 69:480-482 (1979). Rhizoctonia is a particularly problematic plant pathogen for several reasons. First, it is capable of infecting a wide range of crop plants. Second, there are no commercially available chemical fungicides that are effective in controlling the fungus. Because of these circumstances, an inhibitor against R. solani would be of substantial interest as a potential control for this pathogen.
Many biological control Pseudomonas strains produce metabolites, such as antibiotics, that inhibit the growth of fungal pathogens. Howell et al., Phytopathology 69:480-482 (1979); Howell et al. Phytopathology 70:712-715 (1980). These have been implicated in the control of fungal pathogens in the rhizosphere. Several past studies have focused on the effects of mutations that result in the inability of the disease control bacterium or fungus to synthesize these compounds. Kloepper et al., Phytopathology 71: 1020-1024 (1981); Howell et al., Can. J. Microbiol. 29:321-324 (1983). In these cases, the ability of the organism to control the disease was all but eliminated. In particular, Howell et al., Phytopathology 69:480-482 (1979) discloses a strain of Pseudomonas fluorescens which was shown to produce an antibiotic substance that is antagonistic to Rhizoctonia solani.
In Baker et al., Biological Control of Plant Pathogens, (American Phytopathological Society, St. Paul, Minn.)( 1982), pages 61-106, it is reported that an important factor in biological control is the ability of an organism to compete in a given environment. Thus, it is desirable to obtain novel strains of biocontrol agents which effectively control the growth of plant pathogens, particularly fungi, such as Rhizoctonia solani and Pythium ultimum, and are able to aggressively compete with indigenous bacteria and other microflora that exist in the rhizosphere of the plant.